Polymerization processes can generally be classified into two basic types: homogeneous and heterogeneous processes. This classification is usually based on whether the initial reaction mixture or the final reaction mixture or both is homogeneous or heterogeneous. Some polymerization systems which start out as homogeneous may become heterogeneous as the polymerization reaction proceeds, due to the insolubility of the resulting polymer in the polymerization media.
Heterogeneous polymerizations are used extensively as a means to control the thermal and viscosity problems associated with mass and solution polymerizations. Emulsion polymerization is a heterogeneous polymerization process used by industry to polymerize a variety of monomers. The use of a water or water-rich phase in emulsion polymerizations is common. Polymers commonly formed by emulsion polymerization include acrylics, styrenics, polyvinylchloride, styrene-butadiene rubber, ethylene-propylene-diene terpolymer, polystyrene, acrylonitrile-butadiene-styrene copolymers, neoprene rubber, ethylene-vinylacetate copolymers, styrene-maleic anhydride polymers, poly(tetrafluoroethylene), tetrafluoroethylene copolymers, poly(vinylfluoride), and the like.
Heterogenous polymerizations employing a carbon dioxide phase have recently been proposed. Carbon dioxide is a desirable media for polymerization because it is inexpensive and environmentally safe. U.S. Pat. No. 5,312,882 to DeSimone et al. proposes a heterogenous polymerization process for the synthesis of water-insoluble polymers in carbon dioxide. The heterogenous reaction mixture includes carbon dioxide, monomer, and surfactant. The disclosed heterogenous reaction does not include a water or water-rich phase. U.S. Pat. No. 4,933,404 to Beckman et al. proposes a microemulsion polymerization system including a low polarity fluid which is a gas at standard temperature and a second, water phase. The monomer is soluble in the water phase, and is polymerized in the micelles to produce a water soluble polymer.
Carbon dioxide has also been employed in polymerization systems for the polymerization of hydrocarbon and fluorinated monomers. For example, U.S. Pat. No. 3,522,228 to Fukui et al. proposes the polymerization of vinyl monomers using hydrocarbon polymerization initiators in carbon dioxide. U.S. Pat. No. 4,861,845 to Slocum et al. discloses a gas phase polymerization of tetrafluoroethylene and other fluoromonomers diluted with gaseous carbon dioxide. PCT Publication No. WO 93/20116 to the University of North Carolina at Chapel Hill discloses processes for making fluoropolymers which include solubilizing a fluoromonomer in a solvent comprising carbon dioxide. The fluoromonomers are selected from the group consisting of fluoroacrylate monomers, fluoroolefin monomers, fluorostyrene monomers, fluorinated vinyl ether monomers, and fluoroalkylene oxide monomers.
There remains a need in the art for a method of making polymers which avoids the use of expensive or environmentally objectionable solvents and which are relatively easily separable from the polymer produced. In addition, it would be desirable to provide polymerization processes, particularly for the polymerization of fluorinated monomers, which is capable of commercialization in conventional polymerization equipment.